Hb Montefiore (α126(H9)Asp → Tyr). High oxygen affinity and loss of cooperativity secondary to C-terminal disruption

Hb Montefiore was found, in the heterozygous state, in a Puerto Rican female who had a slightly elevated total Hb level. Structural analysis revealed that Asp-α126 was replaced by Tyr. Hb Montefiore migrates close to HbF (at pH 8.6) and accounts for 20.3% of the hemolysate. Oxygen binding of red blood cells revealed a 40% decrease in the P₅₀ (pH 7.4) and a low n value of 1.6 (normal: 2.6). Depletion of red blood cell 2,3-DPG did not change the results. Stripped Hb Montefiore at pH 7.2 showed an 8-fold reduction in P₅₀ (0.6 versus 4.6 mm Hg) and very low cooperativity (n = 1.2 versus 2.9 for the control). Heterotopic effectors, as 2,3-diphosphoglycerate and inositol hexaphosphate had a normal effect and in addition, they increased cooperativity. The chloride ion effect and the Bohr effect were moderately reduced. A bezafibrate derivative (L345), known to bind α126, increases the P₅₀ of HbA by 9-fold, but only by 1.5-fold that of Hb Montefiore. Combining these functional studies with intrinsic fluorescence and Resonance Raman spectroscopy, we interpret the very low n value and the high oxygen affinity for Hb Montefiore as a result of both a destabilized T state that switches to R upon ligand binding and a deoxy T state that binds ligands with higher affinity than that of deoxy HbA. Hb Montefiore still binds ligands cooperatively, but the difference in ligand binding properties of the two quaternary states has been drastically reduced.